In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the mobile station and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased dramatically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instant, an access terminal in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector. In operation, a BTS typically emits a pilot signal on each sector, and a mobile station in receipt of a pilot signal regularly measures the strength (Ec/Io, i.e., energy versus spectral density) of the pilot and notifies the cellular wireless network when the signal strength of the pilot falls above or below designated thresholds.
As the mobile station moves between wireless coverage areas of a wireless communication system, such as between cells or sectors, or when network conditions change or for other reasons, the mobile station may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the mobile station monitoring the signal strength of various nearby available coverage areas, and the mobile station or the BSC (or other controlling network entity) determining when one or more threshold criteria are met. For instance, the mobile station may continuously monitor signal strength from various available sectors and notify the BSC when a given sector has a signal strength that is sufficiently higher than the sector in which the mobile station is currently operating. The BSC may then direct the mobile station to hand off to that other sector.
In some wireless communication systems or markets, a wireless service provider may implement more than one type of air interface protocol. For example, a carrier may support one or another version of CDMA, such as EIA/TIA/IS-2000 Rel. 0, or A (hereafter “IS-2000”) for both circuit-cellular voice and data traffic, as well as a more exclusively packet-data-oriented protocol such as EIA/TIA/IS-856 Rel. 0, A, or other version thereof (hereafter “IS-856”). Access terminals operating in such systems may be capable of communication with either or both protocols, and may further be capable of handing off between them, in addition to being able to hand off between various configurations of coverage areas.
In a typical wireless communication system, communications from a base station (or more specifically, the BTS) to an access terminal are carried on a “forward link” of the air interface, and communications from an access terminal to a base station are carried on a “reverse link” of the air interface. Data sent on both the forward and reverse links may be first assembled into units called frames, which are then encoded for transmission to or from the access terminal at regular intervals (corresponding to a frame rate), typically 20 milliseconds in duration (although other frame intervals can be used). As a result of imperfect transmission, some frames received by the mobile station on the forward link or by the base station on the reverse link may contain errors. Additionally frames may be lost, which can be inferred by the failure to receive a frame during one or more of the regular frame-rate intervals. Other frames—ideally, the majority of them—will be received without errors.
On either link, the receiving entity (e.g., a mobile station or base station) can compute a ratio of (i) the number of error-containing frames received during a given period of time to (ii) the total number of frames received during the same period of time. This ratio, computed by the mobile station on the forward link or by the base station on the reverse link, is called the frame error rate (FER). For either link, the FER is an indicator of the quality of service provided over the respective link. For instance, frame errors may manifest as lost audio samples, which in turn cause choppy or distorted audio output when played out by a receiving device. Similarly, frame errors may represent packet-data loss that may result in retransmissions and lower overall throughput. In general, the higher the FER, the lower the quality of service will be, and vice versa.
Cellular service providers may perform tests on calls made over the network to determine whether the audio fidelity of the calls are up to the service provider's standards. If the audio fidelity of the calls does not meet the service provider's standards, the service provider may repair and/or upgrade components within the network to increase the audio fidelity of the calls. Various methods exist to calculate the audio fidelity of a call over a cellular network. For example, the mean opinion score is a numerical measure for representing audio fidelity. The mean opinion score is expressed as a number in the range 1.0 to 5.0, where 1.0 is lowest perceived quality and 5.0 is the highest perceived quality. The mean opinion score for a call (or group of calls) is typically generated by averaging the results of a set of standard, subjective tests set forth in Recommendation P.800, Telecommunication Standardization Sector of ITU, Geneva, Switzerland, August, 1997, although other calculations for the mean opinion score could be utilized. These subjective tests generally require a person to listen to and rate the voice quality of a call over the cellular communications network. Because of the subjective nature of the tests, obtaining a mean opinion score for calls over a network can be time consuming and expensive.